Many Veterans have lower-limb amputations due to traumatic injuries or complications associated with diabetes and vascular disease. Walking on uneven terrain is unbalanced and uncomfortable for persons with lower-limb amputations because most current ankle-foot prostheses do not adapt their function for different sloped surfaces. This project will examine the advantages of a new adaptable ankle-foot prosthesis that can adapt to different terrain on every step of walking using only passive mechanical parts (i.e., without the need for motors or batteries). This new system also provides plantarflexion in the late stance phase of walking, which should increase the efficiency of walking. To examine the potential benefits of the new adaptable ankle-foot system, twenty Veterans with unilateral transtibial amputations will be recruited for the study. During the first visit, the prosthetic ankle torque versus ankle angle curves will be measured while the Veterans walk on a treadmill at five surface slopes (-10, -5, 0, +5, and +10 degrees). Subjects will also walk up and over a series of ramps to examine first-step adaptation to surface slopes. A load cell in the prosthesis coupled with motion analysis will allow for the measurement of ankle torque vs. ankle angle as the Veterans walk on the various sloped surfaces. It is anticipated that the ankle torque versus ankle angle curves will shift towar dorsiflexion for uphill slopes and toward plantarflexion for downhill slopes when using the adaptable ankle-foot system. These shifts in the ankle torque vs. ankle angle curve are not expected for similar use of a non-adaptable ankle-foot system (Ossur VariFlex with EVO). In the second and third visits, the same Veterans will walk with both the adaptable and non-adaptable ankle- foot prostheses over a range of speeds and surface slopes while their submaximal oxygen uptake is measured. For each condition (prosthesis/speed or prosthesis/slope), subjects will walk for six minutes on a treadmill, followed by at least ten minutes of rest (or longer if needed to return to baseline oxygen uptake). During the second visit, subjects will walk at three speeds (0.75, 1.00, and 1.25 m/s) with both the adaptable and non-adaptable ankle-foot systems (in a random order). During the third visit, subjects will walk with both ankle-foot systems on three slopes (-5, 0, and +5 degrees) in a random order. The adaptable ankle-foot system is expected to reduce submaximal oxygen uptake compared with the non-adaptable ankle-foot system for both speeds and slopes due to its ability to store and release more energy than current passive non- adaptable ankle-foot systems and its ability to adapt its function on every step to changes in surface slope. Durability testing of the adaptable ankle-foot system will be conducted in parallel with research experiments. The adaptable ankle-foot system will be tested using the International Organization of Standards (ISO) 22675 testing protocol. Design revisions will occur throughout the funding period to improve the durability of the adaptable ankle-foot system. The goal of the development activities is to pass the ISO 22675 test of 2,000,000 cycles of testing without failure. The research and development activities planned for the new adaptable ankle-foot system are designed to provide evidence of the system's ability to adapt to different surface slopes on every step of walking as well as to increase walking efficiency. These activities will provide useful information regarding the design approach used by the current system, while improving the chances for commercialization of the technology for Veterans.